The present invention relates to DC-to-DC converters and more specifically to a method and apparatus for determining the value of an unregulated DC bus voltage in an insulated power system.
As well known in the controls art three phase AC voltage provided by a power station at a single 60 Hz frequency can be rectified using an AC-to-DC rectifier. While useful for some applications, such rectified DC voltage is not suitable for other applications. This is because the magnitude of AC voltage provided to a facility routinely fluctuates due to varying grid usage and other power system occurrences and the fluctuating AC voltages cause the resulting DC voltage to fluctuate. The fluctuating DC voltage is said to be unregulated.
One application for which unregulated DC voltage is unacceptable is powering analog and digital electronics which typically require a regulated (i.e. constant within an acceptable range) voltage source.
Switch-mode DC-to-DC converters are used to convert unregulated DC voltage input into regulated DC voltage output. An exemplary converter includes a transformer including primary and secondary windings, a switch, a regulator and a rectifier/filter circuit.
The regulator is linked to the switch to open and close the switch. The switch and primary form a series pair between positive and negative rails of the unregulated DC bus voltage. The secondary is linked to the rectifier/filter circuit. By opening and closing the switch, current from the unregulated source is provided to, and then cut off from, the primary, resulting in a pulsating voltage across the transformer secondary. The rectifier/filter circuit rectifies the pulsating voltage and filters ripple to provide the regulated DC output voltage. By adjusting the switching duty cycle, the DC output voltage magnitude is adjusted and can be kept constant. This type of DC-to-DC regulator is often used to provide regulated DC power to electronics.
In addition to being converted into a regulated DC voltage for powering electronics, another common use for an unregulated DC bus voltage is to feed a DC-to-AC inverter. Inverters are used to convert DC voltage to controllable frequency and amplitude AC voltage. To this end, a controller is typically provided to control inverter switching in a manner calculated to result in a desired amplitude and frequency.
Modern controllers typically include complex electronic configurations which require a regulated DC bus voltage. Therefore, inverter systems also often include a DC-to-DC converter for converting the unregulated DC voltage to a regulated DC source to power the controller.
To determine how to control an inverter to generate an AC voltage having a desired amplitude and frequency, a typical controller considers a large number of operating parameters. One parameter considered essential to operation of most controllers is the unregulated DC bus voltage magnitude. In effect, it is necessary to know inverter input voltage to determine how to control the inverter to provide a desired output.
As indicated above, unregulated DC bus voltage fluctuates and therefore controllers which consider DC bus voltage magnitude require some mechanism to determine bus voltage. A resistive voltage divider and a differential amplifier have been used in many systems to determine the unregulated DC bus voltage magnitude. The divider circuit solution is relatively inexpensive and works well for many applications.
Despite the advantages of such a simple divider circuit, such circuits cannot be used in certain applications. For example, for safety purposes many applications require an insulated power system wherein the unregulated DC bus voltage is monitored to identify any current leakage which may potentially signal a dangerous condition. When a current leak is sensed, the leak is interpreted as a ground fault and the converter/inverter system is prevented from operating. In an insulated power system, even current loss through a voltage divider circuit is sufficient to prevent the insulated system from operating.
Thus, it would be advantageous to have a method and/or apparatus for determining an unregulated DC voltage magnitude in an insulated power system.